West Nile virus causes an infection that can lead to inflammation of the brain (encephalitis), the spinal cord (myelitis), or the tissues surrounding it and the spinal cord (meningitis). If your doctor suspects a mosquito-borne illness, he or she will take a medical history to assess your risk of West Nile virus infection. People who live in or travel to areas where the virus has been found are at risk of West Nile virus infection.

If you have symptoms that West Nile is affecting the brain and spinal cord, your blood will be tested for antibodies to the virus, a sign that you have been infected. The initial blood test screens for immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies to the virus. If the the initial test shows West Nile virus infection, you may have a follow-up test 2 weeks later to see if antibodies are increasing. For more information on the immunoglobulins test, see the medical test topic Immunoglobulins.

False-positive results, which show that you have the virus when in fact you do not, can occur if you have been infected with a similar virus, such as the St. Louis encephalitis virus. However, this does not affect treatment, because the treatment for all forms of encephalitis is similar. A false-positive result may also occur if you have recently received certain vaccinations, such as those for yellow fever and Japanese encephalitis.

If you have symptoms of encephalitis, a lumbar puncture, also called a spinal tap, may be done to look for antibodies and signs of infection in the cerebral spinal fluid, which surrounds the brain and spinal cord. If antibodies are found, another test will be done to look at the virus's genetic material.

In some cases, you may have a magnetic resonance imaging (MRI) test to identify encephalitis or inflammation of the brain and the membranes surrounding the brain and the spinal cord (meningoencephalitis).

Donated blood may be screened with a West Nile virus blood test called Procleix. This blood test detects RNA in donated blood and plasma. Screening donated blood for West Nile virus helps to keep it safe so that people who are receiving the blood do not get the virus.